Use the correct exception when checking for HTTP 406

Tempest has introduced NotAcceptable exception [1], breaking our usage of
UnexpectedResponseCode. Use the new exception.

[1] https://opendev.org/openstack/tempest/commit/e60b31706240d989ba6d0244acbbb97154c47a13

Change-Id: I1e07e09348613493572d6aa55a2a3cde686497ed
2 files changed